The present invention relates to a stainless steel wire particularly for use as the material for springs, and process for manufacturing the same.
As a material for springs, stainless steel wire has better corrosion resistance and better heat resistance than carbon steel wire, but has poorer workability into bearings. The invention of nickel-plated stainless steel wire has obviated the shortcoming and has widened the application of stainless steel wire. On the other hand, springs are made in a wide variety of shapes. In order to prevent springs of similar shapes from mixing, it has become usual to color springs for color identification and decoration.
For the coloring of springs, there are two methods. One is to paint the wire after worked into springs: the other is to paint it before working. With the former method, it is difficult to paint springs uniformly and it is more time-consuming to paint springs one by one. Thus, the latter method has been proposed. In the latter method, before working the wire into springs, a heat-resistant paint is applied thinly on the wire. The paint must be such as to withstand the temperature of annealing done after working into springs to improve the strength of springs. On the other hand, wire for springs is required to have a coating having good adhesiveness and lubricity to stabilize the shape of springs. Also, it is required to have a stable way of curling. A stainless steel wire for springs is generally subjected to drawing after solution heat treatment. The lubricant used in the drawing step remains uniformly on the drawn wire in a thin layer. The remaining lubricant serves as a lubricant in the ext step, that is, working into springs. In this working step, a stable curling is given to the wire. But, if the wire were painted before working into springs, there would be no lubricant remaining on the surface of wire. Therefore, poor lubrication causes wide variation in the shape of springs and peeling of the coating. Also, painting by baking is preferable to achieve a coating having a good adhesiveness. But, high temperature for baking impairs the toughness of wire. Painting the wire which has been elongated by cold-drawing takes a longer time and more labor.
An object of the present invention is to provide a stainless steel wire for springs which is colored, has a good workability into springs, uniform wire diameter, and as good toughness as ordinary stainless steel wire for springs.
Another object of the present invention is to provide a process for manufacturing such a stainless steel wire for springs.